OF TUB STRUCTURE OF WOOD AND HERBS. 77 



line, the eighteenth of an inch in length, no less than one hundred and fifty 

 tubes. In an inch long there would consequently be (18X150) twenty-seven 

 hundred tubes, and in a square inch (2700 X 2700) seven millions two hun- 

 dred and ninety thousand tubes. The examinations of decayed wood, where 

 the tubes were empty, led to the same result ; and further corroboration was 

 obtained by Dr. Hooke from the inspection of petrified wood, where the 

 situation of the pores was very conspicuous. In woods that are remarka- 

 ble for their conSgkctness and density, the vessels or tubes are still smaller and 

 more numerous'6within a given space. The largest tube observed by Hedwig, 

 in the stem of the gourd, possessed an apparent size of one-twelfth of an inch, 

 when seen through a microscope that magnified two hundred and ninety times. 

 Its real diameter was therefore one-twelfth of an inch, diminished two hundred 

 and ninety times : or the three thousand four hundred and eightieth part of an 

 inch. If, therefore, within the extent of a square inch a collection of tubes like 

 these occupied but one half of the space, no less a number than six millions 

 fifty-five thousand two hundred would be comprised within this small compass. 



ARRANGEMENT. These tubes are not arranged singly throughout the trunks 

 and branches, but are collected into small bundles ; in the stems of herbs and 

 of roots, each small bundle is composed of from thirty to one hundred tubes, and 

 sometimes of many hundreds. In herbs the bundles are often placed at conside- 

 rable distances from each other without any symmetrical arrangement, while in 

 trees they are regularly disposed in concentric circles ; and, when cross sections 

 of wood are viewed through a microscope, are seen distinctly arranged in a great 

 variety of the most beautiful and elegant figures. It was supposed by the 

 earlier writers on vegetable anatomy, that the tubes which have just been de- 

 scribed were of two kinds j the office of the first class being to convey sap, and 

 that of the second to carry air to the different parts of the plant. The tubes 

 were thence denominated sap-vessels and air-vessels ; the latter class also receiv- 

 ed the name of spiral vessels, from the peculiar manner in which the tube is 

 formed. It is now however believed by distinguished naturalists, that there are 

 no vessels exclusively employed for the conveyance of air, but that all the tubes 

 found in the woody parts of plants are sap-vessels of one kind only, and that in 

 different plants, and in different circumstances and conditions of vegetation, 

 these vessels or tubes are capable of assuming various forms, sizes, and character- 

 istics ; a circumstance which has led many observers to the belief, that they con- 

 stituted several distinct species, which subserved different purposes conducive to 

 the life and growth of the plant. 



In figures"ll7, 118, 119, 120, 121, and 122, is delineated the sap-tube, under 

 the several forms which at times it assumes. In figure 117, it appears to be 

 covered with rows of small projecting knots. Stripes cross it in figure 118; 

 while in figures 119 and 120, the spiral structure is distinctly seen, the tube being 

 formed in the same way as a paper-lighter is made, by rolling a long and nar- 

 row surface spirally upon itself. In figure 121, the vessel appears to consist of a 



